This invention relates generally to radio communications networks and more particularly to optimizing performance with multipath fading channels. Specifically, a combination of transmit power control and joint space-time diversity is used to optimize the communications links in the entire network.
The capacity of a cellular mobile communications system comprising a network of base stations and mobile stations is limited by the cochannel interference (CCI) and Inter Symbol Interference (ISI). CCI is due to interference caused by users sharing the same channel, and IST is caused by delayed components of a signal when the delay spread in a multipath channel is larger than a fraction of a symbol. It is well known that channel capacity can be increased by using space-time diversity strategies at the base stations to reduce CCI and ICI.
In the uplink (mobile stations transmitting, base stations receiving), adaptive receiver beamforming (space diversity) schemes have been widely used to reduce both CCI and ISI, and to adjust the beam pattern to optimally increase the effective signal to noise ratio (SNR) at tie output of the beamformer. In order to reduce CCI, the beamformer will place nulls at the directions of interference, while the gain at the direction of the desired transmitter is maintained constant. In a single tap diversity, the signal from the main path is considered as the signal of interest, and the beamformer will reject the ISI terms by placing nulls at the directions of multipath signals. However, rejecting the ISI terns by beamforming is an inefficient way of using spatial diversity. A more effective way is to exploit the temporal properties of the multipath signal and reduce ISI by using the delayed versions of the signal in the estimation of the transmitted symbol, and using the beamforming capability of the array for the CCI rejection. Joint implementation of this space-time diversity will optimally reduce CCI and ISI with large delay spread in the wireless channels, resulting in a significant increase in the SNR. Receiver diversity can be implemented independently at each base station receiver, without affecting the performance of other links, using a local feedback from the receiver output to adjust the combining vectors.
In the downlink (base stations transmitting, mobile stations receiving), receive diversity is not practical because it is difficult to deploy antenna arrays at the mobile stations. However, the downlink capacity can be improved using transmit diversity with antenna arrays at the base station transmitters by adjusting the beam pattern of each antenna array to minimize the induced interference to undesired receivers. Transmit diversity is substantially different in nature from receive diversity. Transmit beamforming at a base station transmitter will change the interference to all mobile receivers, and as a result transmit beamfbnning must be done jointly in the entire network. Furthermore, the signal probing has to be done at the mobile stations, while antenna array beam patterns are adjusted at the base stations, except for Time Division Duplex (TDD) systems where uplink and downlink channels are reciprocal and the uplink channel information can be used for downlink. Transmit beamforming by placing nulls at the direction of each cochannel receiver is well adapted to cases where the number of cochannels are less than the number of antenna elements. Such strategies include selective transmission in reciprocal networks which uses the weight vectors calculated by a singe tap diversity combiner at the receiver, rejecting the ISI by placing the beamformer nulls at the directions of the interference sources; adjustment of the transmitter patterns to minimize the overall interference to the other cochannel receivers using single tap transmit diversity; and in multipath networks with large delay spread and having the number of available nulls less than the number of antenna elements, the use of multitap transmit diversity to reject the interference to other links by placing nulls at the directions of the multipath signal to the undesired users.
The link quality is not guaranteed in any of the aforementioned receive or transmit diversity schemes. Furthermore by considering adaptive transmit beamforming between a transmitter and only its receiver, the possible erects that a change of beam pattern in that transmitter could have on all receivers in the entire network are ignored. Finally, use of these schemes does not guarantee a feasible solution for all cochannel links in a network such that the SNR is satisfied at each link.
According to the present invention, joint space-time diversity is combined with transmit power control in both the uplink and do ink in the uplink; the result will be to minimize mobile transmit power while maintaining the SNR in each link above a threshold value. In the down the result will set the SNR at each mobile station to a specified value if a feasible solution exists.
In the present invention joint space-time diversity and transmit power control are combined in networks with multipath channels.
In the uplink case, the mobile power allocation and multitap receive diversity combining weight vectors are calculated such that the mobile transmitted power is minimized, and the SNR at each link is maintained above a threshold. Compared to prior art multitap diversity methods, the link quality is satisfied at each link while the transmitted power is minimized for each transmitter.
In the downlink case, the effect of beamforming at a transmitter is considered within the context of the entire network, and a set of feasible space-time diversity weight vectors and power allocations are jointly found to achieve the required link quality at each link. The beampattern of all the antennas in the network are adjusted jointly such that the SNR at each link is grater than a predetermined threshold, which will favor links with higher interference level.
In a TDD system, where the transmit and receive channels are reciprocal, the uplink weight vectors are used to calculate the downlink power allocations such that the link quality is satisfied at each link. In a Frequency Division Duplex (FDD) system or a TDD system with fading, where the uplink and downlink channels are different, the downlink channel gains are required to calculate the downlink diversity vectors and power allocations. The downlink channel characteristics must be measured at the mobile stations and transmitted to the base station through a feedback channel. In the present invention feasible combining weight vectors are calculated using the global channel measurements, which will result in a feasible solution to the joint problem if there exists any, even in the case that the number of cochannels are larger than the number of array elements.
Instead of estimating the channel response and implementing a matched filter, the received signal can be oversampled at the array input to minimize the estimation error due to the sampling of a continuous time received signal. Oversampling can be applied to the transmit and receive diversity schemes.